Fuel cell stacks are electrochemical devices that produce water and an electrical potential from a fuel, such as a proton source, and an oxidant. Although other fuels and oxidants may and have been used, many conventional fuel cell stacks utilize hydrogen gas as the proton source and oxygen gas, air, or oxygen-enriched air as the oxidant. Fuel cell stacks typically include many fuel cells that are fluidly and electrically coupled together between common end plates. Each fuel cell includes an anode region and a cathode region, separated by an electrolytic membrane. Hydrogen gas is delivered to the anode region, and oxygen gas is delivered to the cathode region. Protons from the hydrogen gas are drawn through the electrolytic membrane to the cathode region, where they react with oxygen to form water. While protons may pass through the membrane, electrons cannot. Instead, the electrons that are liberated from the hydrogen gas travel through an external circuit to form an electric current, which also may be referred to as the electrical output of the fuel cell.
The electrochemical reaction utilized within fuel cell stacks is an exothermic reaction. Thus, the fuel cell stack may be thought of as generating both electrical potential and thermal energy (heat). The electrolytic membranes of some fuel cell systems, such as proton exchange membrane (PEM), or solid polymer fuel cell systems, generally need to be within a range of suitable operating temperatures in order for the electrolytic membranes to function properly for generation of the electrical output. If the membrane is below this range of suitable operating temperatures, the fuel cell may not efficiently produce its electrical output. On the other hand, if the membrane is above this range of suitable temperatures, degradation of the membrane may occur.
Thus, conventional fuel cell systems may utilize a thermal management system to control the temperature of the fuel cell stack. This may include supplying thermal energy to the fuel cell stack under conditions in which the temperature of the fuel cell stack is below the range of suitable operating temperatures and/or removing thermal energy from the fuel cell stack under conditions in which the temperature of the fuel cell stack is above the range of suitable operating temperatures. Cooling of the fuel cell stack may be accomplished by directing a stream of thermal management fluid, such as, for example, ambient air, to, or into thermal contact with, the fuel cell stack and exchanging thermal energy between the fuel cell stack and the thermal management fluid. The flow rate of the thermal management fluid to the fuel cell stack, together with the temperature differential between the fuel cell stack and the thermal management fluid, may impact the rate of thermal energy exchange between the fuel cell stack and the thermal management fluid, and thus the temperature of the fuel cell stack.
For fuel cell stacks that are cooled by an ambient air stream, accurate control of the volumetric flow rate and/or the velocity of the air stream that comes into thermal contact with the fuel cell stack may be important to ensure accurate control of the fuel cell stack temperature. Since the rate of heat generation within the fuel cell stack may be a function of the rate of electrochemical reaction within the fuel cell stack and/or the power output from the fuel cell stack, and the rate of thermal energy transfer between the fuel cell stack and the air stream may be a function of the fuel cell stack temperature, the ambient air temperature, and the flow rate of the air stream to the fuel cell stack, the thermal management system may be designed to provide a broad range of air flow rates and thus ensure accurate fuel cell stack temperature control over the entire thermal energy generation and ambient condition operating ranges under which the fuel cell stack is designed to operate. Thus, a thermal management system that may provide sufficient cooling under worst-case, and/or maximum rated, conditions of maximum fuel cell stack thermal output and high ambient temperatures, as well as under conditions of low fuel cell stack thermal output and low ambient temperatures, may be desirable.
In order to ensure reliable temperature control over a broad range of conditions, the flow rate of the thermal management fluid into thermal contact with the fuel cell stack may be varied over a broad range, such as from between 1% and 100% of the maximum thermal management fluid flow rate. As an illustrative, non-exclusive example, a 5 kW fuel cell stack may operate under conditions in which thermal management fluid flow rates from 50 to 600 standard cubic feet per minute (SCFM) may be desired.
If the thermal management system is unable to provide this broad range of thermal management fluid flow rates, overcooling and/or undercooling of the fuel cell stack may result. Overcooling may cause the temperature of the fuel cell stack to decrease below an acceptable level, resulting in a decrease in fuel cell stack reaction kinetics and/or condensation of liquid water within the fuel cell stack, which may cause flooding of the fuel cell stack. Undercooling may cause the temperature of the fuel cell stack to increase to above an acceptable level, resulting in dehydration of the electrolytic membrane within the fuel cell stack, decreased reactant diffusion through the electrolytic membrane, and/or irreversible damage to fuel cell stack components.
It may be difficult to obtain this broad range of thermal management fluid flow rates without the use of complicated control systems and methods and/or multiple cooling devices. Thus, there exists a need for simple and reliable systems and methods of controlling the flow rate of thermal management fluid to the fuel cell stack under a broad range of operating conditions.